Re: SSD + Rust as raid1

[Dominic Raferd <dominic@timedicer.co.uk>]

On 31/05/2013 09:52, Dominic Raferd wrote:
> On 31/05/2013 08:54, Roman Mamedov wrote:
>> On Fri, 31 May 2013 08:47:00 +0100
>> Dominic Raferd <dominic@timedicer.co.uk> wrote:
>>
>>> This is my idea too (see my OP), but I am concerned about optimisation
>>> (--write-behind, --bitmap and --bitmap-chunk settings) especially for
>>> writes.
>>> --write-behind=16384
>> I think this will not work, you will have to use 16383.
> Oh, OK, so 16383 is the maximum then?
>
>>> --bitmap=/mnt/sda1/write-intent-bitmap.file
>> Save yourself lots of maintenance headache, just use --bitmap=internal
>>
>>> --bitmap-chunk=256M
>> Looks OK.
>>
> Thanks Roman, but the problem with using --bitmap=internal is that, as
> Neil Brown posted here on another topic a while ago, this requires a
> synch write to both devices, and the use-case for which write-behind was
> developed involved an external bitmap. Hence my plan to use external
> bitmap file on a fast (SSD-based) separate partition - minimises any
> slow-down caused by having to maintain the write-intent bitmap file.
>

I would be very grateful if someone could confirm whether, if I set up 
RAID1 and with one of the drives specify --write-mostly 
--write-behind=n, that maximum 'n' is 16383, and also whether it is 
permitted in this configuration to set --bitmap=none and thus avoid the 
overhead of maintaining a write-intent bitmap file? (My thinking is that 
for my  needs the extra safety provided by the bitmap file is overkill 
and the slowing effect (and life-shortening of my SSD) might be more 
significant.) If I have to have a bitmap file, it is presumably faster 
to have a larger chunk size, is the maximum permitted 256M?